We call removing a user from a course unenrolling.
To unenroll a user (student, grader, or admin) navigate to the appropriate roster page, select the actions menu, and then select the unenroll option. These steps are shown below.
Removing a user from a course won't delete their work. As examples:
removing a student from a course won't delete any of their submissions
removing a grader from a course won't delete any submissions they've graded, or comments they've left
Once removed, the user will become inactive. You'll still be able to view their work, but they won't be able to access your course. You also won't be able to upload new submissions from inactive students, or assign submissions to inactive graders.
You can also re-activate a user by adding them back to your course roster.